This invention relates to steam generators used in nuclear reactor power plants. It relates particularly to the means used to introduce the feedwater into the steam generator.
Steam generators used in nuclear reactor power plants are very large heat exchangers where heat from a primary fluid heated by the nuclear reactor is transferred to a secondary fluid (water) which is converted into steam and used to drive a turbine generator. Steam generators are housed inside a tall, generally cylindrical steel shell. A large number of U-shaped heat exchanger tubes are enclosed in the shell and have their ends inserted in holes formed in a horizontal tube sheet or plate near the bottom of the steel shell. The tubes are used to convey the primary fluid which has been heated in the nuclear reactor. The secondary fluid or feedwater used to generate the steam is introduced into steam generator in such a manner that the secondary fluid flows around the outside of the heated tubes thereby converting much of the secondary fluid into steam which is allowed to exit the steam generator through an outlet nozzle at the top of the steel shell.
Many early model steam generators (often called preheat SGs or SGS with economizers) were designed to introduce the secondary fluid or feedwater through a feedwater inlet nozzle positioned near the bottom of the steam generator where it was introduced directly to the tube bundle. The incoming secondary fluid then rose upwardly around the tubes where it was heated and converted into steam. The secondary fluid that was not yet converted into steam was captured by the moisture separators and redirected into the steam generator recirculating pool. Recirculating water from this pool flowed down between the tube bundle wrapper and the inside wall of the steel shell to the bottom of the steam generator where it was directed back into the tube bundle where it then was mixed with additional incoming secondary fluid or feedwater.
Other designs of steam generators (often called feedring SGs or non-preheat SGs) have been designed with the feedwater inlet nozzle positioned in the upper part of the steam generator to introduce the secondary fluid or feedwater into the recirculation pool where it mixes with the recirculating water before flowing down between the tube bundle wrapper and the inside wall of the steel shell to the bottom of the steam generator where it is directed back into the tube bundle and then flows upwardly around the heated tubes.
Nuclear power plants that wish to convert from the style of steam generator having a bottom secondary fluid or feedwater inlet flow introduced directly to the tube bundles (preheat SGs) to the design of steam generator having the secondary fluid or feedwater inlet at the top (feedring SGs) have had to incur great expense and difficulty to convert all the power plant secondary fluid piping, pumping, and control systems to accommodate the design change of steam generator.